This application claims the priority of German patent application 196 53 909.9, the disclosure of which is expressly incorporated by reference herein.
The invention is based on a cylinder head for internal-combustion engines, as known, for example, from German Patent Documents DE 36 03 582 C1 or DE 195 02 342 A1.
Cylinder heads for internal-combustion engines are components which have complicated shapes and are produced by casting. The casting cores for casting the interior surface of the charge cycle ducts have an unavoidable offset with respect to an ideal position of the core which fluctuates from one workpiece to the next. The desired position of the interior surface of the charge cycle valves is determined by the position of the respective pertaining valve opening in the combustion-space-side boundary wall of the cylinder head. Because of the core offset, a machining allowance, which increases toward the valve opening, was provided on the charge cycle ducts in the cast crude state in the area close to the opening, which machining allowance corresponds with respect to its amount on the valve opening at least to the maximally permissible offset amount. This results in a variation of the cutting depth of approximately twice the amount of the offset. This interior-side overmeasure of material will then be machined down such that the interior surface of the charge cycle ducts changes in each case without any offset into the valve opening.
For this purpose, the initially cited documents recommend a machining by means of a forming cutter, in which case the spatial desired contour of the charge cycle ducts is produced by a corresponding spatial guiding of a spherical cutter or disk milling cutter. Although, by means of such a machining, a transition of the interior surface of the duct into the valve opening can be produced which has no offset and is advantageous with respect to the flow, because of the spatial movement of the cutter, the required machining is complicated and, because of the low metal removing engagement of the forming cutter into the workpiece at the respective working point, is also very slow. The spatial guiding of the cutter can be carried out only by means of very expensive processing machines so that the machining results in high investment costs. Nevertheless, because of the long-lasting machining, the productivity is low. In addition, the use of more efficient, higher-stress-withstanding and exchangeable hard-metal blades in spherical or disk milling cutters is not possible at least in the case of the cutter sizes which are suitable in practice.
It is known to produce an offset-free transition of the valve opening into the charge cycle ducts in a cutting manner in that a forming cutter, which, in the vertical center, has the shape of a pointed arch and, in its diameter, corresponds to the inside diameter of the valve opening, is cuttingly dipped from the combustion space side in a straight line and concentrically into the valve opening, in which case the interior surface of the duct wall is also machined. Such a machining requires only a simple spindle movement similar to a drilling operation which can be carried out by a low-cost standard machine. A forming cutter, which is required for this purpose, can also be equipped with exchangeable high-performance blades. In addition, such a cutter engages along the whole circumference in the material to be removed so that a high metal removal rate and therefore an efficient machining is possible. By means of the pointed-arch-shaped contour of the cutting teeth, a transition, which is advantageous for the flow, from the machined to the cast duct surface is produced at least in the area of a course of the duct wall which is concave in the flow direction.
In contrast, individually for each workpiece according to the direction of the respective core offset, a more or less pronounced edge is formed on the opposite duct sides with the convex course of the wall, on which edge the flow will separate. Such flow separations form a cross-sectional narrowing and therefore impair an optimal charge cycle. Particularly in the case of the inlet ports, separation-causing machining edges would reduce an optimal cylinder charge or impair an inlet flow which should be constantly good for all engines of a manufacture and thus reduce the engine output which can in fact be achieved by means of the internal-combustion engine or reduce the quality of the combustion.
It is an object of the invention to improve the cylinder head of the above-mentioned type such that the machining allowance in the area of the charge cycle ducts close to the valve can, on the one hand, be worked off without any offset by means of a forming cutter which is guided in a straight line but that nevertheless a surface course can be obtained also in the area of the convexly extending wall parts which is advantageous to the flow.
This and other objects have been achieved according to the present invention by providing a cast cylinder head for an internal-combustion engine, comprising a combustion-space-side boundary wall defining at least one circular valve opening, and a duct wall defining a charge cycle duct communicating with the at least one valve opening, an interior surface of the charge cycle duct being offset relative to a peripheral surface of the boundary wall defining the valve opening within a defined, maximally acceptable tolerance field, said duct wall defining a machining allowance which increases toward the valve opening and which, at the valve opening, corresponds in size to at least a maximally acceptable amount of offset, an area of the charge cycle duct proximate the opening being cuttingly machined on an interior side such that the interior surface of the charge cycle duct changes in each case without offset into the valve opening, wherein a compensating groove is provided proximate the machining allowance of the charge cycle duct in the cast crude condition, extending in the circumferential direction and having a flat V-shaped cross-section, flanks of said groove converging approximately in the center of the groove acutely with respect to a base of the groove, said flanks having a convexly shaped cross-section and being curved, the compensating groove being provided at least in the area of a convex interior wall of said charge cycle duct, the compensating groove extending above one of an upper edge of a recess for a valve seat ring and a valve seat in the valve opening a distance of approximately 20 to 30% of the inside diameter of the valve opening, the groove base being situated outside an ideal course of the interior surface so that not only is there no machining allowance in this area but the real interior surface of the charge cycle duct locally has an overmeasure with respect to said ideal course.
This and other objects have been achieved according to the present invention by providing a crude cast component to be finished into a cylinder head for an internal-combustion engine, comprising: a boundary wall defining a valve opening; and a duct wall having an interior surface defining a duct communicating with said valve opening, at least a portion of said interior surface being convexly curved proximate said valve opening along a longitudinal course of said duct, said interior surface of the duct wall defining a compensating groove proximate said valve opening, said compensating groove having a base extending circumferentially around at least a portion of said convexly curved portion, said compensating groove having a first flank extending convexly from said base toward said valve opening to define a machining allowance in said duct wall, said compensating groove having a second flank extending convexly from said base opposite said valve opening.
This and other objects have been achieved according to the present invention by providing a casting mold for forming a crude cast component to be finished into a cylinder head for an internal-combustion engine, comprising at least one negative mold part having spaces which define: a boundary wall defining a valve opening; and a duct wall having an interior surface defining a duct communicating with said valve opening, at least a portion of said interior surface being convexly curved proximate said valve opening along a longitudinal course of said duct, said interior surface of the duct wall defining a compensating groove proximate said valve opening, said compensating groove having a base extending circumferentially around at least a portion of said convexly curved portion, said compensating groove having a first flank extending convexly from said base toward said valve opening to define a machining allowance in said duct wall, said compensating groove having a second flank extending convexly from said base opposite said valve opening.
This and other objects have been achieved according to the present invention by providing a method of forming a cylinder head for an internal-combustion engine, comprising the step of casting a component including: a boundary wall defining a valve opening; and a duct wall having an interior surface defining a duct communicating with said valve opening, at least a portion of said interior surface being convexly curved proximate said valve opening along a longitudinal course of said duct, said interior surface of the duct wall defining a compensating groove proximate said valve opening, said compensating groove having a base extending circumferentially around at least a portion of said convexly curved portion, said compensating groove having a first flank extending convexly from said base toward said valve opening to define a machining allowance in said duct wall, said compensating groove having a second flank extending convexly from said base opposite said valve opening.
As the result of the compensating groove according to the invention, the machining allowance can be machined in a manner which is advantageous for the flow by means of a forming cutter of a pointed-arch-shaped contour which dips linearly into the valve opening. In the case of a casting core which is placed in the absolutely accurate position which happens relatively infrequently a slight impairment of the flow with respect to the best construction is caused which is, however, within the tolerance range and is accepted. However, instead the achievable duct contours in the case of a moderate to extreme core offset are much better than in the state of the art. The compensating groove according to the invention will in every case avoid a constriction of the duct at a critical point. Thus, while the machining of the charge cycle duct is simple, the invention achieves a low-loss flow and therefore a good cylinder charge.
The compensating groove can be produced virtually without additional costs during the casting operation. The casting cores for the charge cycle ducts must only be provided with a circumferential build-up which negatively corresponds to the groove shape.
The advantage of the invention is a simple, efficient and low-cost machining of the duct end with a simultaneous reduction of the variation of the flow losses in the charge cycle ducts. In particular, in the case of two-way valve engines, the so-called tumbling--a circulation flow of the gases flowing into the combustion space with an axis of rotation situated in the longitudinal direction of the engine--which reacts very sensitively to changes of the flow conditions, is reduced to an easily tolerable fluctuation range despite occurring manufacture-caused fluctuations, so that the tumbling values indicated by the engine designer can also be maintained by means of a simple machining. The selected intensity of the tumbling in an interaction with other combustion-relevant engine parameters is responsible for an orderly and specifically desired combustion sequence. An unacceptably pronounced and uncontrolled change of the tumbling disturbs the compromise selected for a certain engine layout and has a negative effect on the combustion sound and/or the exhaust gas values.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.